1. Field of the Invention
The subject invention relates to a boss or protrusion integrally formed with a part, and more particularly to a metal insert positioned within the boss to receive a fastener when the part is mounted to a surface.
2. Description of the Prior Art
Generally, plastics have comparatively high coefficient of thermal expansion, whereas metals and their alloys have a low coefficient of thermal expansion. The difference between the coefficient of thermal expansion creates a problem when various plastic parts are connected to metal parts. The difference between the coefficient of thermal expansion results in cracks, sink marks, and other problems, particularly in spoilers, roof racks, and the like that are exposed to high temperatures during summer and low temperatures during winter.
Various methods are known for making and mounting spoilers, roof racks, and the like, to a roof of a vehicle. Generally, such spoilers, roof racks, and the like have spaced rails and bosses, wherein the rails and bosses further include end holes which align with round holes through the roof or other parts of the vehicle body at predetermined locations. U.S. Pat. No. 4,446,185 to Waragai et al., U.S. Pat. No. 5,200,252 to Kelman, and U.S. Pat. No. 6,096,256 to Aretz show different fastening bosses and the methods of making them.
The metal inserts were designed to give the boss added structural integrity and to remove heat generated during formation of the boss and foam layer to thereby reduce cracking of the boss and localized blistering and delimitation of outer skin of the part. While trying to reduce cracking of the boss and to localize blistering of the outer skin, the prior art methods failed to provide the part with “Class A” surface, due to the differences in thermal expansion of the different materials. Gaps remain between the part and the insert installed therewithin.